introduction
As LED production costs decline, their use is becoming more common, covering applications ranging from handheld devices to in-vehicles to architectural lighting. The high reliability of LEDs (lifetime over 50,000 hours), high efficiency (>120 lumens/watt) and near-instantaneous responsiveness make it an attractive source of light. Compared to the response time of an incandescent bulb of 200 mS, the LED will illuminate in a short 5 ns response time. Therefore, they are currently widely used in brake lights in the automotive industry.
Drive LED
Driving LEDs is not without challenges. The adjustable brightness requires a constant current to drive the LED and it must be kept constant regardless of the input voltage. This is more challenging than simply connecting an incandescent bulb to a battery to power it.
The LED has a forward VI characteristic similar to a diode. Below the LED turn-on threshold (the white LED's turn-on voltage threshold is approximately 3.5V), the current through the LED is very small. Above this threshold, the current will increase exponentially in the form of a forward voltage. This allows the LED to be shaped as a voltage source with a series resistor with a warning that the model is only valid at a single operating DC current. If the DC current in the LED changes, the resistance of the model should also change to reflect the new operating current. At large forward currents, power dissipation in the LEDs can cause the device to heat up, which will change the forward voltage drop and dynamic impedance. It is very important to fully consider the heat dissipation environment when determining the LED impedance.
When driving LEDs through a buck regulator, the LEDs often conduct the AC ripple current and DC current of the inductor based on the selected output filter arrangement. This not only increases the RMS amplitude of the current in the LED, but also increases its power consumption. This will increase the junction temperature and have a significant impact on the life of the LED. If we set a 70% light output limit as the life of the LED, then the life of the LED will be extended from 15,000 hours at 74 degrees Celsius to 40,000 hours at 63 degrees Celsius. The power loss of the LED is determined by multiplying the LED resistance by the square of the RMS current plus the average current multiplied by the forward voltage drop. Since the junction temperature can be determined by the average power consumption, even a large ripple current has little effect on power consumption. For example, in a buck converter, the peak-to-peak ripple current equal to the DC output current (Ipk-pk = Iout) increases the total power loss by no more than 10%. If you exceed the above loss level, you need to reduce the AC ripple current from the power supply to keep the junction temperature and operating life constant. A very useful rule of thumb is that for every 10 degrees Celsius drop in junction temperature, semiconductor lifetime will be doubled. In fact, most designs tend to have lower ripple currents due to the suppression of the inductor. In addition, the peak current in the LED should not exceed the maximum safe operating current rating specified by the manufacturer.
Topology selection
The information shown in Table 1 helps to select the best switching topology for the LED driver . In addition to these topologies, you can use simple current limiting resistors or linear regulators to drive LEDs, but such methods typically waste too much power. All relevant design parameters include input voltage range, number of LEDs driven, LED current, isolation, EMI suppression, and efficiency. Most LED driver circuits fall into the following topological types: buck, boost, buck-boost, SEPIC, and flyback.
Table 1 Alternative LED Power Topology
Figure 1 shows three basic power supply topology examples. The buck regulator shown in the first diagram is suitable for situations where the output voltage is always less than the input voltage. In Figure 1, the buck regulator controls the current into the LED by changing the turn-on time of the MOSFET. Current sensing can be obtained by measuring the voltage across the resistor, where the resistor should be in series with the LED. An important design challenge for this approach is how to drive the MOSFET. From a cost performance perspective, N-channel field effect transistors (FETs) that require floating gate drive are recommended. This requires a drive transformer or floating drive circuit (which can be used to maintain the internal voltage above the input voltage).
Figure 1 also shows an alternative buck regulator (buck#2). In this circuit, the MOSFET drives the ground, which greatly reduces the driver circuit requirements. The circuit can optionally sense the LED current by monitoring the FET current or a current sense resistor in series with the LED. The latter requires a level shifting circuit to obtain information on the power ground, but this complicates the simple design. In addition, a boost converter is shown in Figure 1, which can be used when the output voltage is always greater than the input voltage. This topology is easy to design because the MOSFET is driven to ground and the current sense resistor is also grounded. One disadvantage of this circuit is that there is no limit to the current through the inductor during a short circuit. You can add fault protection in the form of a fuse or electronic circuit breaker. In addition, some of the more complex topologies provide this type of protection.
Figure 1 Simple Buck and Boost Topologies Power LEDs
Figure 2 shows two buck-boost circuits that can be used when the input voltage and output voltage are high and low. Both have the same trade-off characteristics (where the tradeoff can be seen in two buck topologies for current sense resistors and gate drive positions). The buck-boost topology in Figure 2 shows the gate drive for a ground reference. It requires a level shifted current sense signal, but the reverse buck-boost circuit has a ground referenced current sense and level shifted gate drive. If the control IC is associated with a negative output and the current sense resistor and LED are interchangeable, the reverse buck-boost circuit can be configured in a very useful manner. With proper control ICs, the output current can be measured directly and the MOSFET can be driven directly.
Figure 2. Buck-Boost Topology Adjusts Input Voltage Greater Than or Less Than Vout
One drawback of this buck-boost method is that the current is quite high. For example, when the input and output voltages are the same, the inductor and power switch currents are twice the output current. This has a negative impact on efficiency and power consumption. In many cases, the "buck or boost" topology in Figure 3 will alleviate these problems. In this circuit, the buck power stage is followed by a boost. If the input voltage is higher than the output voltage, the buck stage will be voltage regulated when the boost stage is just energized. If the input voltage is less than the output voltage, the boost stage is regulated and the buck stage is powered. Some overlap is usually reserved for boost and buck operations, so there is no dead band when moving from one model to another.
When the input and output voltages are nearly equal, the benefit of this circuit is that the switching and inductor currents are also nearly identical to the output current. The inductor ripple current also tends to become smaller. Even with four power switches in the circuit, efficiency is typically significantly improved, which is critical in battery applications. The SEPIC topology is also shown in Figure 3, which requires fewer FETs but requires more passive components. The benefit is a simple ground reference FET driver and control circuitry. In addition, the dual inductors can be combined into a single coupled inductor, saving space and cost. But like a buck-boost topology, it has a higher switching current than the "buck or boost" and ripple output currents, which requires the capacitor to pass a larger RMS current.
Figure 3 Buck or boost and SEPIC topology provide higher efficiency
For safety reasons, it may be stipulated to use isolation between the off-line voltage and the output voltage. The most cost-effective solution for this application is the flyback converter (see Figure 4). It requires the least number of components in all isolated topologies. The transformer turns ratio can be designed as a buck, boost or buck-boost output voltage, which provides great design flexibility. But the downside is that the power transformer is usually a custom component. In addition, there is a high component stress in the FET and the input and output capacitors. In stable lighting applications, power factor correction (PFC) can be achieved by using a "slow" feedback control loop that adjusts the LED current in phase with the input voltage. A higher power factor is achieved by adjusting the average LED current required and the input current in phase with the input voltage.
Figure 4 Flyback converter provides isolation and power factor correction
Dimming technology
It is quite common to need to dim LEDs. For example, it may be necessary to adjust the display or adjust the brightness of the architectural light. There are two ways to do this: reduce the LED current or turn the LED on quickly and then turn it off, then let the eye eventually get balanced. Since the light output is not completely linear with the current, the method of reducing the current is the least efficient. In addition, LED chromatography typically changes when the current is below the rated value. Remember that people's perception of brightness is exponentially multiplied, so dimming requires a larger percentage change in current. Because at 3% of the full current, the 3% adjustment error can be amplified to 30% or more under 10% load due to circuit tolerance, which can have a significant impact on circuit design. Although there is a problem with response speed, it is still more accurate to adjust the current by pulse width modulation (PWM). When lighting and displaying, PWM above 100Hz is required to make the human eye not notice the flicker. The 10% pulse width is in the millisecond range and requires the power supply to have a bandwidth above 10 kHz.
in conclusion
As shown in Table 2, the use of LEDs in many applications is becoming increasingly common. It will use a variety of power topologies to support these applications. Typically, the input voltage, output voltage, and isolation requirements will dictate the correct choice. When the input voltage is always high and low compared to the output voltage, buck or boost may be the obvious choice. However, when the relationship between the input and output voltages is not so suppressed, the choice becomes more difficult, and many factors, including efficiency, cost, and reliability, need to be weighed.
Table 2 Many LED applications specify multiple power topologies
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